The present invention is directed to mass transfer devices and, more particularly, to vapor-liquid contact trays and methods of making and utilizing such trays.
Various types of trays are commonly used in mass transfer columns to promote contact and mass transfer between ascending and downwardly flowing fluid streams. The ascending fluid is typically vapor and the descending fluid is typically liquid, although liquid-liquid and gas-liquid systems are also known. Each tray normally extends horizontally across substantially the entire horizontal cross section of the column and is supported around its perimeter by a ring welded to the inner surface of the circular column wall or shell. A number of trays are positioned in this manner with a uniform vertical spacing between adjacent trays. The trays may be located in only a portion of the column to perform one part of a multi-step process occurring with the column. Alternatively, the trays may fill substantially the entire open area within the column.
Trays of the type described above contain one or more downcomers that are positioned at openings in the tray deck to provide passageways for liquid to descend from one tray to an adjacent lower tray. Prior to entering the downcomer, the liquid on the tray deck interacts with ascending vapor that passes through openings provided in selected portions of the tray deck. Those areas of the tray deck containing vapor openings are commonly referred to as “active” areas because of the vapor and liquid mixing and frothing that occurs above those areas of the tray. The downcomer inlets on vertically adjacent trays may extend in laterally opposite directions to promote more uniform lateral distribution of the descending liquid or they may extend in a parallel but offset relationship.
At least two broad categories of trays are used in mass transfer columns. The first category of trays is referred to as counter-flow trays or multiple downcomer trays and is characterized by a relatively large number of downcomers positioned on each tray. In counter-flow trays, the downcomers are generally trough-shaped and extend above the tray so that liquid must accumulate on the tray surface before entering the inlet to the downcomer. The lower outlet of such downcomers utilizes a perforated plate that causes liquid to accumulate within the downcomer to seal the outlet against the entry of ascending vapor into the downcomer where it would impede the descending flow of liquid. The lower outlet of this type of downcomer is normally located well above the deck of the tray below and above the inlet of the downcomers on the tray below.
The second broad category of trays is referred to as cross-flow trays and differs from counter-flow trays in several significant respects. First, the number of downcomers utilized in cross-flow trays is normally less than the number used in counter-flow trays. Cross-flow trays normally use a side downcomer located at opposite ends of adjacent trays, although in multiple pass configurations one or more additional downcomers are located at intermediate locations on each cross-flow tray. Downcomers on cross-flow trays normally discharge liquid onto an imperforate receiving pan located on the tray below and inlet weirs are provided adjacent the receiving pan to cause liquid to accumulate above the level of the downcomer outlet to seal against entry of vapor into the downcomer. By contrast, as discussed above, counter-flow tray downcomers use a perforated discharge plate rather than an inlet weir to form the liquid seal and liquid is discharged onto the active area of the underlying tray rather than onto an imperforate seal pan.
Counter-flow and cross-flow trays also differ in the manner in which they are constructed and installed in a column. Counter-flow trays are normally supported entirely on the support rings that are welded to the inner surface of the column shell. Because the downcomer outlets in counter-flow trays are located well above the tray below, some variations in tray spacing resulting from deviations in ring placement and downcomer construction can be accommodated without significantly affecting tray performance. As a result, each downcomer and tray panel in a counter-flow tray is normally preassembled and then simply carried into the column through a manway or other opening and positioned on the support ring.
By contrast, the downcomers in cross-flow trays extend almost completely to the tray below and must be carefully positioned to ensure that the proper vertical clearance is provided between the lower edge of the downcomer and the imperforate seal pan on the tray below. Variations in this vertical clearance can significantly affect the amount of liquid that can be discharged from the downcomer and may result in detrimental flooding of the downcomer and column. Because the vertical clearance is so important, the downcomer is normally assembled within the column using multiple pieces so that the lower wall or apron of the downcomer can be adjusted to provide the proper vertical clearance.
The downcomers in counter-flow and cross-flow trays also differ in the manner in which they are supported in the column. Counter-flow downcomers are box-like in configuration and normally do not extend completely across the column cross section in order to simplify installation of the trays. Because fewer downcomers are normally used in cross-flow trays, they traditionally extend completely across the horizontal cross section of the column to maximize the total liquid carrying capacity of the downcomers. Normally the column shell forms the short end walls of these downcomers and, in the case of side downcomers, one of the longitudinally extending downcomer side walls. The remaining side walls of each downcomer are attached to the column shell using numerous vertically extending and inclined bolting bars that are prepositioned and welded to the inner surface of the column shell. Location and installation of these bolting bars is a time-consuming and labor-intensive task that can add significantly to the installation cost of cross-flow trays. Moreover, if the bolting bars are misaligned, installation of the downcomer is made more difficult and/or the important vertical discharge clearance at the downcomer outlet may be adversely affected.
A need has thus developed for a cross-flow tray that is easier to install and does not require the use of downcomer bolting bars.